Reading a little further down in the spec, there are two parameters 'user' and 'key'. User is listed as 'optional' and 'key' is "mandatory only if 'user' has been specified". The spec links to their keygen page, so give it a try.

I suspect that the user-id is used under the covers and isn't truly optional. Otherwise my updates would step all over your data as soon as there is more than one user.